The invention relates to a measuring device for determining the content of magnetizable substances, particularly ferrite and margensite, in a sample, the measuring device including an exciting coil which preferably has an iron core, for producing a magnetic field to be applied to the sample, and further including an evaluation circuit and a means for displaying measurement quantities proportional to the content of magnetizable substances in the sample.
The content (proportion) of magnetizable substances in given samples or workpieces can be determined by known methods such as metallographic examination, X-ray diffraction, testing the saturation magnetization, application of magnetic scales (balances), calculations from analysis of the Schaeffler diagram, use of a magnetic ferrite measuring device employing the eddy current principle such as the Type 1053 from the firm of Foerster, or the Type M 10 B from the firm of Fischer, and by other conventional methods.
The content of magnetizable substances, particularly the content of ferrite as a structural constituent thereof which may be undesirable or may be required in rather large percentage amounts depending on the type of material, must often be determined reliably, for quality control reasons. For practical applications, the measurement method should be free of susceptibility to interference, should be rapid, and should be applicable to, e.g., solid samples.
Metallographic, X-ray diffraction, magnetic scale, and saturation magnetization measurement methods are cumbersome, subject to interference, and often completely unusable for particular samples. Known portable measuring devices enable interference-free ferrite measurements, but for higher ferrite contents they are susceptible to experiencing systematic error.
A known ferrite-measuring device (that from the firm of Foerster) is comprised of a device with a contacting element for engaging the sample, operating elements, and display means. The contacting element is a ferromagnetic rod which bears two coils. One coil (primary coil) is excited by an a.c. current and induces an EMF in the secondary coil (secondary winding), according to the law of induction EQU e=k.w.f. .PHI..
When the rod is applied to a sample with a particular ferrite content, the magnetic flux is changed correspondingly. Because the other parameters are unchanged, an EMF proportional to the ferrite content of the sample is generated.
By appropriate calibration, this EMF can be made proportional to the ferrite content of the probe, within a particular narrow range of correlated values.
However, in order to produce a substantial measurement signal, it is necessary to have a substantial change in the magnetic flux .PHI., because of the transformer law (i.e., Faraday's law). And becuase .PHI. is a function of B (magnetic induction, as may be seen from FIG. 1), and the relative permeability .mu..sub.r is linked with B, the operating region of this measuring device lies in the low field strength region (.ltoreq.4 A/cm). However, this is the region of high slope of the magnetization lines, hence high .mu..sub.r, and as a result of the high curve slope the measurement accuracy is still quite high. Nonetheless, as shown in FIG. 2, at the relatively low field strengths of known measuring devices there can occur intersection of the characteristic curves of the relative permeability .mu..sub.r, which can lead to errors in the registered values.
At relatively high frequencies of the exciting magnetic field of the measuring probe (e.g., 2 kHz), the measured values are influenced by the electrical conductivity of the sample material. As studies have shown, these errors (resulting from eddy currents induced in the sample by the exciting field) are negligible only at frequencies .ltoreq.500 Hz. The eddy current losses increase with conductivity and frequency, and have an undesirable effect on the measuring circuit.
It is thus an underlying object of the present invention to provide a measuring device wherein the electrical conductivity of the sample material has a negligible effect on the measurement results, and whereby the contents (proportions) of magnetizable substances between 0% and 100% can be determined accurately. In addition, errors relating to engagement of the sample by the device should be kept low, and the effects of magnetization present in the sample should be substantially eliminated. In addition, it is desirable for the field strength of the exciting magnet to intersect the magnetization lines beyond the bend in the curve as shown in FIG. 2, where the trend of the magnetization curves has a regular relation to the content of magnetizable substances.
These objects are met by the measuring device of the present invention in which the exciting coil is a part of an oscillation circuit tuned to a specific resonant frequency and preferably operated at a selected operating point; further, the magnetic parameters of the exciting coil, particularly its inductance, are variable by means of the magnetic field, which variation determines a new resonant frequency of the oscillation circuit; and still further, the variation of at least one of the parameters of the oscillation circuit (e.g., current, voltage, and/or frequency), which variation is associated with a variation in the resonant frequency of the oscillation circuit, is expressed as a measurement quantity which is proportional to the content of magnetizable substances.
The measuring device according to the present invention enables the content of magnetizable substances in the entire range (0-100%) to be determined very accurately, because the exciting coil is part of a tuned resonant circuit, and the resonant frequency of a resonant circuit reacts very sensitively to variations in its parameters. A simple calibration measurement can be performed with the exciting coil not in contact with the sample, whereby the null point of the measurement is exactly established. The end point of the scale is established using a sample comprised 100% of a magnetizable substance (e.g. pure ferrite).
According to a preferred embodiment of the invention, the oscillation circuit comprises the exciting coil, a capacitor, and a coupling coil for introducing the operating frequency of the oscillation circuit, with the exciting coil, capacitor, and coupling coil being connected in a series resonant connection. It is advantageous under this arrangement if the value selected as the measurement value for the evaluation circuit is the voltage drop across the exciting coil. According to an alternative embodiment, the oscillation circuit comprises the exciting coil, a capacitor, and a coupling coil for introducing the operating frequency of the oscillation circuit, connected in a parallel resonant circuit, wherein the value selected as the measurement value for the evaluation circuit is the rate of increase of the voltage in the oscillation circuit, or the voltage drop across a resistance inserted between the capacitor and the exciting coil. The embodiments are of simple construction, durable in operation, relatively temperature-insensitive, and accurate in measurement.
One can obtain large measurement ranges of the measurement values obtained from the resonant circuit if the operating frequency selected for the series or parallel resonant circuit is disposed in the frequency range of maximum slope on a side (flank) of the resonance curve.
Another embodiment of the invention is characterized in that an oscillation circuit is comprised of the exciting coil and a capacitor in parallel or in series with it, which circuit is coupled to a LF (low frequency) generator and/or forms a part of the LF generator, wherein the frequency of the oscillation circuit is variable due to interaction with the sample. It is provided that the oscillation circuit is tuned to a specific resonant frequency, and the measurement value employed for the evaluation circuit is the frequency variation occurring in the oscillation circuit when the inductance of the exciting coil is changed. This measurement device of complex design can be made very sensitive, and can provide very accurate measurement results.
Preferably, the field strength of the magnetic field applied to the sample from the exciting coil is between 20 and 200 A/cm, particularly between 30 and 100 A/cm. When magnetic fields of this magnitude are employed, errors attributable to details of the engagement of the sample by the contacting element do now play as great a role as when weaker magnetic fields are employed. In addition, ordinary residual magnetization in the sample is erased or rendered of negligible effect on the measurement. Also, the lines of magnetization measured are in a regular pattern with regard to different magnetizable substances (e.g. ferromagnetic, ferritic, etc.).
In applying such magnetic fields, it is advantageous if the exciting coil surrounds a contacting core, preferably ferritic, which is to engage the sample, and if the exciting coil is in turn surrounded by a preferably cylindrical magnet yoke, with one end of this yoke connected with the contacting core, and the contacting core extending beyond the other end of the yoke. This enables a convenient contacting element to be constructed which can make good contact with the sample, and the magnetic flux of the contacting element can be caused to accurately and substantially engage the sample.
For evaluating the signal, it is a simple matter to insert a bridge rectifier and a bandpass filter as the evaluation circuit for the measurement signal ahead of the display means.